Major World Rivers Research Articles

SHIFT: a spatial-heterogeneity improvement in DEM-based mapping of global geomorphic floodplains

Abstract. Floodplains are a vital part of the global riverine system. Among all the global floodplain delineation strategies empowered by remote sensing, digital elevation model (DEM)-based delineation is considered to be computationally efficient with relatively low uncertainties, but the parsimonious model struggles with incorporating the basin-level spatial heterogeneity of the hydrological and geomorphic influences into the map. In this study, we propose a globally applicable thresholding scheme for DEM-based floodplain delineation to improve the representation of spatial heterogeneity. Specifically, we develop a stepwise approach to estimate the floodplain hydraulic geometry (FHG) scaling parameters for river basins worldwide at the scale of the level-3 HydroBASINS to best respect the scaling law while approximating the spatial extent of two publicly available global flood maps derived from hydrodynamic modeling. The estimated FHG exponent exhibits a significant positive relationship with the basins' hydroclimatic conditions, particularly in 33 of the world's major river basins, indicating the ability of the approach to capture fingerprints from heterogeneous hydrological and geomorphic influences. Based on the spatially varying FHG parameters, a ∼ 90 m resolution global floodplain map named the Spatial Heterogeneity Improved Floodplain by Terrain analysis (SHIFT) is delineated, which takes the hydrologically corrected MERIT Hydro dataset as the DEM inputs and the height above nearest drainage (HAND) as the terrain attribute. Our results demonstrate that SHIFT validates better with reference maps than both hydrodynamic-modeling- and DEM-based approaches with universal parameters. The improved delineation mainly includes better differentiation between main streams and tributaries in major basins and a more comprehensive representation of stream networks in aggregated river basins. SHIFT estimates the global floodplain area to be 9.91×106 km2, representing 6.6 % of the world's total land area. SHIFT data layers are available at two spatial resolutions (90 m and 1 km), along with the updated parameters, at https://doi.org/10.5281/zenodo.11835133 (Zheng et al., 2024). We anticipate that SHIFT will be used to support applications requiring boundary delineations of the global geomorphic floodplains.

Light Rare Earth Elements in Freshly Deposited River Sediments of Ganga Alluvial Plain, Northern India: Geogenic Variability and Anthropogenic Influences

ABSTRACT Rare Earth Elements (REE) are the industrial “Vitamins” because of the high-technology based modern uses and their anthropogenic inputs in natural environment, this study mainly focused on Geogenic variability and anthropogenic imprints of Light Rare Earth Elements (LREE), in the recently deposited sediments of the Ganga Alluvial Plain. All the Sediment samples were analyzed by using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). LREE comprises about ~91% of total REE in the Gomati River Sediments and Cerium (300 µg/g) was the most abundant LREE in the bedload sediments. Average LREE concentrations in bedload (170 µg/g) and suspended load (184 µg/g) sediments of the Gomati River were observed higher than the Average Sediments and the World Major Rivers Suspended Sediments and therefore, characterizing the LREE contamination in sediments. This contamination level assessed on the basis of Potential Ecological Risk Assessment (PERI), Contamination Factors (CF), Degree of Contamination (CD) and Geo-accumulation Index (Igeo), the trend of element contamination showed that Sm>La>Pr>Ce>Nd and PERI shows that LREE pose moderate to strong ecological risk, mostly by Pr and Sm in the study area. The spatial and temporal enrichment of LREE in sediments were observed in the Basin since last two decades. This paper mainly highlights the effects of socio-economic development linked modern high-tech processes on LREE contamination in the river sediments and cycling by the sub-tropical fluvial environment of southern Asia; where rivers supply ~30% of the global sediment input to the world’s ocean.

The flux and fate of plastic in the world's major rivers: Modelling spatial and temporal variability

The flux and fate of plastic in the world's major rivers: Modelling spatial and temporal variability

Channel Water Storage Anomaly: A New Remotely Sensed Quantity for Global River Analysis

AbstractRiver channels store large volumes of water globally, critically impacting ecological and biogeochemical processes. Despite the importance of river channel storage, there is not yet an observational constraint on this quantity. We introduce a 26‐year record of entirely remotely sensed volumetric channel water storage (CWS) change on 26 major world rivers. We find mainstem volumetric CWS climatology amplitude (CA) represents an appreciable amount of basin‐wide terrestrial water storage variability (median 2.78%, range 0.04%–12.54% across world rivers), despite mainstem rivers themselves represent an average of just 0.2% of basin area. We find that two global river routing schemes coupled with land surface models reasonably approximate CA (within ±50%) in only 11.5% (CaMa‐Flood) and 30.7% (HyMap) of rivers considered. These findings demonstrate volumetric CWS is a useful quantity for assessing global hydrological model performance, and for advancing understanding of spatial patterns in global hydrology.

Trends and prospects in the Yangtze River Basin research: A bibliometric analysis

AbstractThe management and protection of the world's major river basins have received increasing attention, and a series of related measures, including fishing bans and ecological engineering restoration, has been pioneered in the Yangtze River Basin (YRB). This paper provides a quantitative and qualitative analysis of research trends and hotspots in the water environment of the YRB through bibliometric means. The database period was from its inception to December 31, 2020. This review combines the changing trends in the water environment of the YRB in the form of keywords. The results show that the governance of the YRB is based on the relationship between “water” and “sediment” as an entry point, focusing on changes in the “temporal” and “spatial” allocation of water resources; further, when considering the problem of “eutrophication” caused by the release of “nitrogen” and “phosphorus” from “sediments,” water quality factors such as “heavy metal pollution” and “organic pollution” are becoming more prominent; then water quality factor affects “biodiversity,” aquatic ecology becomes more and more concerned, and “ecological risk” factor is taken into consideration. Therefore, future management of the YRB requires consideration of all elements of the basin, including water resources, water quality, and aquatic ecology, and how to coordinate these all material elements is the obstacle and breakthrough point. In addition, further studies on research frontiers have revealed that ecological risk, emerging bioinformatics technologies, and, source apportionment will become the new and popular directions and trends in research. This review is intended to provide insights into the management of other large river basins around the world.

Distribution and spatial-temporal variation of organic matter along the Yangtze River-ocean continuum

River-ocean continuums are rich environments key to the transformations of organic matter and nutrients from many sources. Human impacts on these ecosystems can be local, upstream, or global. Particulate organic matter provides one tool for exploring these processes; inventories and carbon and nitrogen stable isotopes are useful indicators. This study conducted comprehensive field investigations from the upper reaches of the Yangtze River to the coastal waters to explore the distribution and spatial-temporal variation of particulate organic carbon (POC) and nitrogen (PON), and dissolved organic carbon (DOC) in surface waters from April–May (Spring) and October–November (Autumn) 2014. Carbon and nitrogen isotopes (δ13C and δ15 N) were used to assess organic matter sources. Numerous environmental factors were also measured. PON and POC exhibited similar spatial and temporal variations along the river mainstream. POC increased from the upper to lower reaches, similarly in both seasons; POC declined seaward from the river mouth. In contrast, PON showed greater seasonal and spatial variations along the continuum, with greater primary production in offshore areas. DOC along the continuum was higher in autumn, likely related to greater river discharge and warmer waters. The DOC/POC ratios by weight were >1 in most samples except for the ones collected from the turbidity maximum zone, suggesting that DOC contributed to a larger fraction of the total organic matter inventories. End-member mixing models revealed that the proportion of organic matter from upstream (allochthonous) sources was >70% in the mainstream during both seasons. In coastal waters, the proportion of allochthonous sources was only >57%. Compared with other major world rivers, DOC and allochthonous POC are especially high in the Yangtze River-ocean continuum. Human activities causing variations in terrestrial inputs are likely an important driving factor. In addition, the contribution of soil erosion caused by urbanization to riverine organic carbon cannot be ignored.

Assessing the long-term planform dynamics of Ganges–Jamuna confluence with the aid of remote sensing and GIS

The confluence of the Ganges, Jamuna, and Padma rivers is one of the most dynamic in the world, an internationally important research area because of the confluence of two of the world's major rivers, the Ganges and the Brahmaputra. Morphological changes in this area have resulted in severe erosion along the banks. Riverbank erosion is one of Bangladesh's most critical issues, requiring a substantial solution. Riverbank erosion affects millions of people in Bangladesh each year because of erosion in this confluence zone. Consequently, understanding the morphological shifting pattern of the confluence is crucial. The research aims to quantify actual bank shifting near the confluence of the Ganges, Jamuna, and Padma rivers in terms of shifting rate and area during a 25-year period (1990–2015). The acquired satellite images were geo-referenced, and bank lines were digitized to carry out this research. The bank line is the linear construction that separates the outside boundary of the river channel from the floodplains. To assess channel width fluctuation, the distance between the extreme borders of the left and right banks, including mid-channel sandbars, was measured. This period is split into five stages, each lasting 5 years, to measure the maturity of change. Furthermore, the long-term trend from 1972 to 2015 is qualitatively discernible. Landsat satellite pictures were used to investigate this morphological change. The study provides current and reliable information on the planform dynamics of the Ganga–Jamuna confluence. This study will help design and implement drainage development plans and erosion control techniques in this crucial confluence zone.

Anthropogenic Influences on an Urban River: Differences in Cations and Nutrients along an Urban/Suburban Transect

The Anacostia River in Washington, D.C. has been experiencing the challenges typical of urban rivers over the last 70+ years. Here, we examine six years (2014 to 2020) of base-flow geochemistry of three tidal Anacostia sites and three suburban sites. Parameters examined include pH, hardness, SAR, alkalinity, TDS, Ca, Mg, Na, K, Fe, Mn, Zn, Al, As, B, Ba, Be, Cd, Cr, Cu, Co, Mo, Ni, Pb, total P, S, Sr, Ti, NO3−, and NH4+. Not surprisingly, winter and spring months showed very high Na (means of 786 mg/L and 1000 mg/L, respectively). Plotting Na/(Na + Ca) versus TDS shows contributions from groundwater, but also differences from major world rivers. Main stem locations usually had Ca/Sr ratios > 200, suggesting that concrete was the source of Ca; however, suburban sites showed high Ca as well and suburban Ca/Sr ratios were frequently <200, indicating a different source for Ca. Most sites showed low median Si:NO3 ratios (between 3 and 5), suggesting elevated NO3− from non-natural sources. The data are consistent with freshwater salinization syndrome (a specific type of urban stream syndrome), and also show that the developed landscape in suburban environments influences geochemistry differently than in urban environments.

A Strontium and Hydro-Geochemical Perspective on Human Impacted Tributary of the Mekong River Basin: Sources Identification, Fluxes, and CO2 Consumption

As the largest and most representative tributary of the Mekong River, the Mun River Basin (MRB) provides critical understanding of regional hydro-geochemical features and rock weathering processes on a basin scale. The present study measured strontium (Sr) isotopes with hydro-geochemistry data of 56 water samples in detail in the MRB in northeast Thailand. The dissolved Sr contents and 87Sr/86Sr isotopic ratios were reported to be 8.7–344.6 μg/L (average 126.9 μg/L) and 0.7085–0.7281 (average 0.7156), respectively. The concentrations of dissolved Sr in the mainstream slightly decreased from upstream to downstream, while the variation trend of 87Sr/86Sr was on the contrary. Correlation analysis showed that Na+ strongly correlated with Cl− (0.995, p < 0.01), while Ca2+ exhibited weak relationships with SO42− (0.356, p < 0.01). Samples of the MRB exhibited lower Mg2+/Na+, Ca2+/Na+, HCO3−/Na+ and 1000Sr/Na ratios, and gathered around the end-member of evaporite dissolution, with slight shift to silicate weathering end-member, demonstrating the dominant contribution of evaporite dissolution and silicate weathering on dissolved loads. Comparing with data of major world rivers from previous research, our results remained consistency with rivers draining through similar geological conditions. The dissolved Sr flux to the adjacent Mekong River was estimated to be 20.7 tons/year. In accordance with the forward model, silicate weathering rate and CO2 consumption rate during dry season were calculated to be 0.73 tons/km2/year and 1.94 × 104 mol/km2/year, and may get underestimated due to intense water consumption by extensive agricultural activities. The superimposed effect of anthropogenic impacts on the water environment could enhance chemical weathering, and thus should be taken into account in regional ion cycles and carbon budgets. These findings highlight the coupling analysis of Sr isotopes and hydro-geochemistry in Earth surface processes and provide basic investigation for sustainable regional water treatment mechanisms in the pan basin of the Mekong River.

Coupling between physical processes and biogeochemistry of suspended particles over the inner shelf mud in the East China Sea

This study examines how geochemical signals regarding the source and transformation in suspended particles are transported in the marine portion of the sediment dispersal system. In the source-to-sink sedimentary continuum of a world major river, the Changjiang (Yangtze) River, is used to demonstrate the close coupling between the marine physical processes and the geochemical signals carried by suspended particles. A sediment trap mooring configured with a non-sequential sediment trap, a downward-looking ADCP, CTD, LISST-100× were deployed for 29 days in the winter of 2014 off the coast of Zhejiang Province, China in water depth of 22.5 m. The mooring rendered two types of data. Physical processes included temporal changes of water depth, currents at 11 depths, temperature at two depths, and salinity at one depth. There was 15-cm accumulation of captured particles in the trap, which rendered the total mass flux of 233.3 g/cm2/day. The trap material was analyzed for grain-size composition, total 210Pb, minerals including mica/illite, kaolinite, chlorite, k-feldspar, plagioclase, quartz, and dolomite. Organic material (TOC, TN) and carbon isotope δ13C were also measured. The range of terrestrial fraction (Ft) of the trap material was between 45–60%. EOF analysis was used to examine the covariability in both data sets. For the physical data set, 37 variables were used in the analysis. For the sediment trap material, 15 variables were used. The EOF results of the physical processes indicate the tidal flow was the most important physical forcing for the transport of suspended particles and water masses that carried marine- and terrestrial-sourced particles. Wave and current combined resuspension entrained reworked sediment off the seabed, which was then transported landward or seaward. The most important covariability of the particle properties in the sediment trap was caused by hydrodynamic sorting that separated the clay from silt and sand sizes. Geochemical signals of the particles were carried by clay and the particle mass (dry weight) was associated with silt and sand. The temporal characteristics of the sorting show the spring-neap tidal cycle. The next major distinction was the contrast between terrestrial- and marine-sourced materials. The EOF analysis further distinguish contributions from the distal Changjiang River and proximal small rivers at the tertiary level of the covariability. The coverabilities in both data sets show close correspondence. Our findings give firm evidence to show marine processes dominated the transport of land-derived fluvial sediment and the incorporation of marine-sourced particles into the dispersal system. There is complexity in both physical processes and geochemical signals of suspended particles. However, our study points out the close coupling between the two.

Rare earth element behavior in springs and streams on a basaltic island: San Cristóbal, Galápagos

Dissolved Rare Earth Element (REE) fractionation mechanisms in freshwater environments have been parameterized by laboratory experiments and measured in major world rivers. To investigate the relationship between such observations and small-scale natural systems, we analyze REE contents in water samples from groundwater springs and streams on San Cristóbal Island, Galápagos. The island is comprised of a single lithology (Ocean Island Basalt) and exhibits a steep precipitation gradient on the windward slope over a short spatial extent, which results in a wide range of physicochemical properties and regolith weathering intensities. This allows us to investigate primary controls of REE fractionation without the influence from varying lithology. Samples show a wide range of measured pH, as well as concentrations of Mn and Fe. Local basalt-normalized REE patterns exhibit a wide range ([Nd/Yb]b = 0.19–1.21 and [Ce/Ce*]b = 0.65–2.22) for the small areal extent of the watersheds (~13 km2). Comparisons between data from 0.45 μm to 0.02 μm filtered samples demonstrate the importance of colloidal-sized (0.02–0.45 μm) metal species in aqueous solution. Our results suggest that two mechanisms are responsible for the observed REE fractionation in this catchment: pH-driven changes in aqueous complex stability by organic or anionic complexes and the preferential adsorption of heavy rare earth elements (HREE) onto Fe- and Mn- oxyhydroxides at low pH. This contrasts with moderate enrichment of light rare earth elements (LREE) observed at higher pH. These results suggest that several proposed REE fractionation mechanisms established previously in laboratory experiments can be observed in a natural watershed system, despite being obscured in larger world rivers. The homogenous lithology and strong climate gradient of San Cristóbal island allow for the observation of REE fractionation mechanisms which are usually obscured in larger world rivers despite being predicted in laboratory experiments, such as a positive Ce anomaly associated with HREE enrichment in low pH waters.

Triple Oxygen Isotopes in Evolving Continental Crust, Granites, and Clastic Sediments

This Chapter considers triple oxygen isotope variations and their 4 Gyr temporal evolution in bulk siliciclastic sedimentary rocks and in granites. The d18O and D'17O values provide new insights into weathering in the modern and ancient hydrosphere and coeval crustal petrogenesis. We make use of the known geological events and processes that affect the rock cycle: supercontinent assembly and breakup that influence continent-scale and global climate, the fraction of the exposed crust undergoing weathering, and isotopic values of precipitation. New data from a 5000 m Texas drillhole into the Oligocene Frio Formation demonstrate minimal isotopic shifts from mudrocks to shales during diagenesis, mostly related to expulsion of water from smectite-rich loosely cemented sediment and its conversion to illite-rich shale. Inversion of triple oxygen isotope fractionations return isotopic values and temperatures along the hole depth that are more consistent with weathering conditions in the Oligocene and modern North America (d18O = -7 to -15‰, and T of +15 to +45°C) rather than d18O from 8 to 10‰ diagenetic water in the drill hole at 175-195°C. More precise T and d18Owater are obtained where the chemical index of alteration (CIA) based detrital contribution is subtracted from these sediments. Triple oxygen isotopes from suspended sediments in major world rivers record conditions (T and d18Ow) of their watersheds, and not the composition of bedrock because weathering is water-dominated. In parallel, the Chapter presents new analyses of 100 granites, orthogneisses, migmatites, tonalite-trondhjemite-granodiorite (TTG), and large-volume ignimbrites from around the world that range in age from 4 Ga to modern. Most studied granites are orogenic and anatectic in origin and represent large volume remelting/assimilation of shales and other metasediments; the most crustal and high-d18O of these are thus reflect and record the average composition of evolving continental crust. Granites also develop a significant progressive increase in d18O values from 6-7‰ (4-2.5 Ga) to 10-13‰ (~1.8-1.2 Ga) after which d18O stays constant or even decreases. More importantly, we observe a moderate -0.03‰ step-wise decrease in D'17O between 2.1 and 2.5 Ga, which is about half of the step-wise decrease observed in shales over this time interval. We suggest that granites, as well as shales, record the significant advent and greater volumetric appearance of low-D'17O, high-d18O weathering products (shales) altered by meteoric waters upon rapid emergence of large land masses at ~2.4 Ga, although consider alternative interpretations. These weathering products were incorporated into abundant 2.0-1.8 Ga orogens around the world, where upon remelting, they passed their isotopic signature to the granites. We further observe the dichotomy of high-D'17O Archean shales, and unusually low-D'17O Archean granites. We attribute this to greater contribution from shallow crustal hydrothermal contribution to shales in greenstone belts, while granites in the earliest 3.0-4.0 Ga crust and TTGs require involvement of hydrothermal products with lower-D'17O signatures at moderately high-d18O, which we attribute to secondary silicification of their protoliths before partial melting. The Chapter further discusses evolution of the shale record through geologic history and discusses the step-wise change in d18O and D'17O values at Archean/Proterozoic transition. Denser coverage for shales in the past 1 billion years permits investigation of the rocks and their weathering in the last supercontinent cycle, with observed lighter d18O values, characteristic for the mid-Phanerozoic at the initiation of Gondwana breakup. The continuing increase in d18O values of the shales since 4 Ga is interpreted to reflect accumulation of weathering products via shale accretion to continents, as low-density and buoyant shales tend to not subduct back into the mantle. The rock cycle passes triple oxygen isotopic signatures from precipitation to sedimentary, metasedimentary, and finally to anatectic igneous rocks. Continental crust became progressively heavier in d18O, lighter in D'17O due to incremental accumulation of high-d18O sediments in accretionary wedges. Second-order trends in d18O and D'17O are due to supercontinent cycles and glacial episodes.

Categories of Vendian catchments – sources of fine-grained aluminosiliciclastic materials for the Serebryanka and Sylvitsa group deposits (Middle Urals)

Research subject. The article discusses a number of geochemical features (distribution of lanthanides and Th, values of (La/Yb)N and Eu/Eu*) of fine-grained clastic/clay rocks of various Vendian regional stages of the western slope of the Middle Urals. The results were interpreted in the context of data on the composition of the catchment areas of a number of modern rivers and information on the distribution of the same characteristics in suspended matter and modern bottom sediments of the “downstream North Dvina River–White Sea”.Methods and materials. The content of La, Sm, Eu, Gd, Yb, and Th in as many as 200 samples of shales and mudstones of the Serebryanka and Sylvitsa groups served as a research material. The research method was an analysis of the localization of individual and average data points of clayey rocks of the Laplandian, Redkinian, Belomorian and Kotlinian regional stages on paired diagrams (La/Yb)N–Eu/Eu* and (La/Yb)N–Th developed on the basis of analytical data on the composition of bottom sediments of estuarine parts of different categories/classes of modern rivers. Results. It was shown that the sedimentary sequences of the western slope of the Middle Urals are mainly composed of fine-grained clastic material belonging to categories 1 (sediments of world major rivers) and 2 (sediments of rivers draining sedimentary/mixed sedimentary formations). This material is likely to have entered the sedimentation area from the east, from the growing Kadomian Varanger-Kanin-Timan folded-thrust belt/Timan orogen, or/and from the west (Riphean sedimentary sequences of Kama-Belsk aulacogen). In addition, the igneous and metamorphic rocks of the East European Platform basement composing the structures, which could be classified as “igneous/metamorphic terranes” under modern classification, were likely to have made a certain contribution. Conclusions. The obtained results agree well with the conclusions obtained earlier as a result of mineralogical and petrographic studies of the conglomerates and sandstones of the Serebryanka and Sylvitsa groups, as well as with the data from a previous geochemical analysis of clay rocks.

Edmond Halley's Contributions to Hydrogeology.

Edmond Halley's Contributions to Hydrogeology.

Dissolved potassium isotopic composition of major world rivers

High-precision potassium (K) isotope ratios have recently been proposed as a new tool for tracing continental weathering and reconstructing Earth’s past climates. The premise is that the K isotopic composition of seawater is sensitive to terrestrial weathering changes. Modern seawater (δ41KNIST SRM3141a = +0.12 ± 0.07‰) is significantly enriched in heavy K isotopes compared to the Bulk Silicate Earth (BSE) and the Upper Continental Crust (UCC). However, the controls causing such a large isotopic fractionation between these two major reservoirs are not well understood. Dissolved K in river water is one of the major inputs of K to seawater. To constrain the poorly defined K isotopic composition of riverine input to the global ocean and to understand the controlling factors of the K isotope composition of seawater, we analyzed the K isotopic composition of 32 river samples from 24 major rivers globally. These rivers drain all continents except Antarctica and collectively account for 40% of the annual global river discharge and 39% of the total K flux into the ocean. We observed a large range in K isotopic composition across all the rivers analyzed, ranging from δ41K = −0.59 ± 0.04‰ to −0.08 ± 0.04‰, but found no significant K isotopic variations among samples collected from the same river under differing flow conditions. We attribute the dissolved K isotopic composition of global rivers to the fraction of K retained in clay minerals during chemical weathering. Isotopically light K is retained with the clay fraction during weathering leading to heavy isotope enrichment in the dissolved K load relative to the BSE and UCC. The flux-weighted and regionally-adjusted mean composition of all rivers studied here (−0.38 ± 0.04‰) serves as a global estimate of the riverine δ41K value. The seawater K isotopic composition (i.e., +0.12 ± 0.07‰) cannot be explained solely by the riverine input. Other mechanisms (hydrothermal input, reverse weathering, biological fractionation) are needed to explain seawater K isotopic composition.

Application of newly developed ensemble machine learning models for daily suspended sediment load prediction and related uncertainty analysis

ABSTRACT Ensemble machine learning models have been widely used in hydro-systems modeling as robust prediction tools that combine multiple decision trees. In this study, three newly developed ensemble machine learning models, namely gradient boost regression (GBR), AdaBoost regression (ABR) and random forest regression (RFR) are proposed for prediction of suspended sediment load (SSL), and their prediction performance and related uncertainty are assessed. The SSL of the Mississippi River, which is one of the major world rivers and is significantly affected by sedimentation, is predicted based on daily values of river discharge (Q) and suspended sediment concentration (SSC). Based on performance metrics and visualization, the RFR model shows a slight lead in prediction performance. The uncertainty analysis also indicates that the input variable combination has more impact on the obtained predictions than the model structure selection.

Triple oxygen isotope investigation of fine-grained sediments from major world's rivers: Insights into weathering processes and global fluxes into the hydrosphere

Continental weathering is accompanied by formation of clays and other secondary minerals and their δ18O and Δ17O values should hence reflect to some extent signatures of meteoric water (δ18OMW) and mean annual temperatures (MAT). Our ability to extract climate information from weathered products across the geologic history relies on analytical methods tested and calibrated against modern climate conditions. We here present triple-oxygen isotope analyses of clay-size sediments from 45 rivers worldwide, as well as δ18O analyses of corresponding silt- and sand-rich detrital fractions, which altogether cover about 25% of the continental area that drained into the oceans, extending from the tropics to polar regions. The majority of studied clays closely approximate weathering products, always having high-δ18O signatures regardless of the bedrock type, and in equilibrium with local meteoric waters. Silts are only ∼1.9‰ lighter on average due to greater detrital dilution. Overall, bulk clays from across different climatic regions do not vary much isotopically; an observation which we attribute to opposing effects of temperature on clay-water fractionation and hydrologic relationship between temperature and δ18OMW. Mathematical inversion of measured clay δ18O and Δ17O values (corrected for detrital contribution) into MAT and δ18OMW, compiled for each studied watershed, returns satisfactory estimates. Globally, triple O isotopes in clays appear to be water-dominated, being controlled almost exclusively by δ18OMW at respective temperature of weathering, with minor effects related to evaporation. Using sand from rivers, correlation of δ18O silts with detrital proportions, and estimated surface outcrop of different rock types, we additionally arrive at a +11.5‰ estimate for the exposed silicate crust undergoing weathering.Globally-averaged, sediment-flux weighted clay δ18O and Δ17O values are +14.80‰ and −0.164‰, respectively. These values are significantly skewed toward O isotope signatures for the southeast Asia and western Pacific regions, characterized by very high sediment fluxes to the ocean. Using both clay- and silt-size fractions, the total weighted silicate weathering δ18O signature exported to the world's ocean is −2.59‰, almost 50% higher the previous estimate, yielding an ice-free world hydrosphere estimate of −0.78‰.Overall, the modern river clays represent a snapshot of modern weathering conditions on continents, and associated first-order climatic signatures related to MAT and δ18O of the hydrosphere. This implies that measured increase in δ18O and stepwise decrease in Δ17O in shales in the geologic record capture: evolving global hydrologic cycle upon continental emergence, decrease in global MAT or diagenetic conditions, and decreasing ocean mass via rehydrating of the mantle by subduction of hydrated low-δ18O, high-Δ17O slabs.

Agricultural innovation and environmental change on the floodplains of the Congo River

Climate‐driven environmental changes bring new risks but also opportunities to populations living along the world's major rivers. Based on ethnoecological fieldwork, in this paper we examine how people living in the cuvette centrale of the Congo basin have adopted flood‐recession agriculture on islands in the Congo River, taking advantage of a secular shift since the 1980s in the hydrological regime of the Congo River. Analyses of the hydrological data reveal that this shift decreased flood risk and significantly extended the growing season on the islands, long enough to enable cultivation of fast‐maturing varieties of manioc and other crops. Flood‐recession farming on islands in the river is today not only an important source of food, but also a source of income for women, who are primarily responsible for seasonal cultivation of fields during the low‐water season. Hydrological changes alone are insufficient to explain the adoption of the new agricultural practice; adoption also arose as a result of dynamic interactions among river fishing, trading, and broader socio‐economic forces. Climate‐change models project an increased frequency of extreme floods. Our results suggest that this change may limit island cultivation in the future. More generally, our findings point to the importance of looking beyond single‐factor, solely environmental explanations in studies of climate‐change adaptation.

Immobilization of relic anthropogenic dissolved organic matter from alpine rivers in the Himalayan-Tibetan Plateau in winter

The Tibetan Plateau is a critical ecosystem that sensitively responds to ongoing glacier shrinkage and permafrost thaw. Dissolved organic matter (DOM) in Tibetan Alpine rivers plays pivotal roles in the biogeochemical cycling of elements and nutrients at regional and even global scales, impacting water quality, downstream environments, and climate. However, little is known about the characteristics and dynamics of DOM in these watersheds. We investigated five major Himalayan rivers in the southern Tibetan Plateau, utilizing bulk dissolved organic carbon (DOC), optical properties, and molecular formulas. We found extremely low DOC and fluorescent DOM (FDOM) levels in the rivers (average DOC: 0.25–0.87 mg L−1, FDOM: 0.02–0.05 RU) with a highly degraded molecular signature, which was enriched with heteroatomic molecular formulas (S-containing: 58–72%, N-containing: 61–86%) and condensed aromatics (31–39% vs. ∼11% in world major rivers). Further, 81–99% of the condensed aromatics was identified as dissolved black nitrogen (DBN) with multiple nitrogen atoms, typical for grassy biomass combustion. The findings highlighted potentially fast DOM remineralization leading to the release of CO2 and enriched apparently anthropogenic condensed aromatics and heteroatomic formulas in what have been considered pristine Tibetan rivers. These findings should be considered in future biogeochemical models and ecosystem management.

Export fluxes of dissolved inorganic carbon to the northern Indian Ocean from the Indian monsoonal rivers

Abstract. Rivers are an important source of dissolved inorganic carbon (DIC) to the adjacent coastal waters. In order to examine the spatial variability in the distribution and major sources of DIC in the Indian monsoonal rivers and to quantify their export flux to the northern Indian Ocean, 27 major and medium-sized rivers were sampled during the discharge period. Significant spatial variability in concentrations of DIC (3.4–73.6 mg L−1) was observed, and it is attributed to spatial variations in the precipitation pattern, the size of rivers, pollution and lithology of the catchments. The stable isotopic composition of DIC (δ13CDIC) also showed strong spatial variability (−13.0 ‰ to −1.4 ‰) in the Indian monsoonal rivers with relatively depleted δ13CDIC values in rivers of the northwest of India (-11.1±2.3 ‰) and enriched values in the southeast of India (-3.5±2.3 ‰). Results of the linear least-squares regression models of Keeling and Miller–Tan's plots indicated that the chemical weathering of carbonate and silicate minerals by soil CO2 is the major source of DIC in the Indian monsoonal rivers. Spatial variability in the deviation of δ13CDIC from the approximated δ13C of the source may probably be due to dominant autotrophic production in rivers of the southeastern region, whereas heterotrophic decomposition of organic matter largely influences the other Indian monsoonal rivers. It is estimated that the Indian monsoonal rivers annually export ∼10.3 Tg of DIC to the northern Indian Ocean, of which the major fraction (75 %) enters into the Bay of Bengal, and the remaining fraction reaches to the Arabian Sea. This is consistent with the freshwater flux, which is 3 times higher for the Bay of Bengal (∼378 km3 yr−1) than for the Arabian Sea (122 km3 yr−1). Despite discharge from the Indian monsoonal rivers accounting for only 1.3 % of the global freshwater discharge, they disproportionately export 2.5 % of the total DIC exported by the world's major rivers. Despite rivers from the region in the southwest (SW) of India exporting DIC that is an order of magnitude lower (0.3 Tg yr−1) than the rivers from other regions of India, the highest yield of DIC was found in the rivers of the SW region of India. It is attributed to intense precipitation (∼3000 mm), favorable natural vegetation of tropical moist deciduous and tropical wet evergreen and semi-evergreen forests, tropical wet climate, high soil organic carbon, and the dominance of red loamy soils in catchments of the rivers of the SW region.